added API/Error class to hold error keys as constants

- re-added Setting::getAll() to API/Setting::set() in response
- updated settings/setup tests

lib/API/Error.php

@@ -0,0 +1,11 @@
+<?php
+namespace MailPoet\API;
+
+if(!defined('ABSPATH')) exit;
+
+final class Error {
+  const UNKNOWN = 'unknown';
+  const BAD_REQUEST = 'bad_request';
+
+  private function __construct() {}
+}